Significant variations in the twinning rate can be observed both in the long term and in the short term. Attempts to explain such variation have been based variously on coital frequency, declining sperm counts, a fertility advantage in twin-prone women, variation in early loss of embryos, and other phenomena. Statistical findings in this project complement endocrinological research and implicate social and psychological influences on gonadotrophic hormone levels. Since the latter control estrogen and progesterone secretion, this phenomenon links social events to health and fertility in ways that will be explored further.